Abstract : A key challenge of future mobile communications research is to strike an attractive compromise between wireless network's area spectral efficiency and energy efficiency. This necessitates a clean–slate approach to wireless system design. This trend is irreversible and will have a profound impact on both the theory and practice of future green heterogeneous networks, which will no longer be purely optimized for approaching the attainable capacity, but will explicitly include the energy efficiency during the design and optimization of the entire protocol stack. This implies that new performance metrics quantifying the energy efficiency of mobile networks are necessary. This will inevitably introduce new fundamental trade-offs that have to be accurately investigated. The objective of this chapter is to summarize the main efforts of the research community in the definition of these energy-efficiency metrics as well as to describe the most important and fundamental trade-offs that emerge with these new performance metrics at hand.